This is a copy and paste documentation from the vim-wiki
A. Find each occurrence of 'foo' (in all lines), and replace it with 'bar'.
:%s/foo/bar/g
B. Find each occurrence of 'foo' (in the current line only), and replace it with 'bar'.
:s/foo/bar/g
C. Change each 'foo' to 'bar', but ask for confirmation first.
:%s/foo/bar/gc
D. Change only whole words exactly matching 'foo' to 'bar'; ask for confirmation.
:%s/\<foo\>/bar/gc
E. Change each 'foo' (case insensitive due to the i flag) to 'bar'; ask for confirmation.
:%s/foo/bar/gci
F. This may be wanted after using :set noignorecase to make searches case sensitive (the default).
:%s/foo\c/bar/gc
is the same because \c makes the search case insensitive.
G. Change each 'foo' (case sensitive due to the I flag) to 'bar'; ask for confirmation.
:%s/foo/bar/gcI
H. This may be wanted after using :set ignorecase to make searches case insensitive.
:%s/foo\C/bar/gc
is the same because \C makes the search case sensitive.